I keep marveling at all the electonic/radio/digital/etc. technology around, and I know there is enough room on the spectrum to handle all the radio waves and digital transmissions (or is there?).
But there ARE quite an awful lot nowadays. (cell phones, regular radio and digital radio, digital tv, and all the little transmissions like cord free computers and phones, wireless sound systems, etc etc etc). It seems if you are an electonica-phile, you’re home would be (if you could see them) in a pulsating, throbbing, twisting mass of wavelength frequencies.
Is there a good analogy to help try and explain this, so one can wrap their head around all these frequencies?
Are there too many? Are there too few?
What’s a wavelength frequency?
The wavelength and frequency of em radiation are inversely proportional to each other (λ = c/f). The frequency of em radiation is a continious spectrum.
They’re all here for your perusal:
Along these same lines, something that’s never been quite clear with me, is the idea of a faraday cage or a satellite dish with holes in etc… I know that a EM wave can’t go through something with holes smaller then it’s amplitude, but I’ve never quite understood why. It seems like part of the wave should bounce back and part of it (the part over the hole) should continue through it. Assume the entire wave stays in tact, then it makes sense. Like thowing a slinky at a screen door. But what makes a wave stay together like a slinky or a spring as opposed to like an occilating (spelling, I know) spray or water. If you shoot water out of a hose and you move the hose back and forth you get a wave like design. Now if there is something in the way, whatever water hits the object will bounce back (kinda) and the rest just continues, nothing holds it all together. Does this make sense?
Frequencies and wavelengths are best described by looking at ripples on a pond. The wavelength is the distance between ripples, and the frequency is how many ripples go by a given spot in a second. If you can imagine holding something and splashing it into the water, you can make patterns in the ripples. For example, splashing for a second, being quiet for a second, then splashing for another second could be a code for something. By varying your codes, you can put all kinds of information in the ripples. The thing is, you can’t splash faster than the ripples will go out across the water. Basically, you can’t carry information that varies any more than about half of the frequency of the wave you are sending it on (this is called Nyquist’s theorem). This is what limits how much information can be sent on a given frequency.
If you vary how big you make the ripples, this is called Amplitude Modulation, or AM. If you vary how fast you make the ripples, this is called Frequency Modulation. You’ve probably heard of these terms before, since radio bands have been specifically set aside for voice transmissions using these methods, and the radio bands have over the years taken on the name of the type of modulation used. There’s nothing particularly special about the type of modulation though. You could easily use FM modulation on frequencies that we would consider to be in the AM band. It’s just that the FCC has restricted those bands to using only those types of signals. Your television, for example, uses FM to carry the voice and AM to carry the picture.
Most of the things you mention are just variations on how to encode data onto radio waves. It’s all the same thing. The FCC sets up various bands of frequencies that you can use for certain things, so that all of the different things agree and can talk to each other. A radio built in the US might not work at all in other areas of the world, because their radio stations aren’t governed by the FCC and might have different frequencies assigned to them.
This web site: http://www.ntia.doc.gov/osmhome/allochrt.pdf (warning, PDF file) has the frequency allocations for the United States. Other parts of the world will be different.
The one thing you mentioned that is slightly different is a cordless keyboard. These use infra-red light. It’s still the same thing. Infra-red light, just like visible light, is just another frequency of electromagnetic radiation, just like radio waves and x-rays. Your TV remote control also uses infra-red light. It’s still exactly the same thing, the signal you want to transmit is encoded onto the light, the same way that signals are encoded onto radio waves.
Most of the frequencies that we can do anything useful with have been allocated for something, so if you come out with some new whiz bang device (like say digital radio) then the FCC may have to change some of its frequency allocations to let it be commonly used by the public.
[QUOTE=Joey P]
Along these same lines, something that’s never been quite clear with me, is the idea of a faraday cage or a satellite dish with holes in etc… QUOTE]
The Faraday cage is a little different from a satellite dish with holes in it. In the presence of an electric field, the conductor that a faraday cage is made up of will be all at the same potential. At the same time, the field inside that cage will be zero. This means that inside the conductor, the voltage is constant. Because of this, no current can flow. This is why your car is a safe place to be in lightning storm, also why people will put these cages around their computers.
As for the satellite dish with holes, I have to do a little research to explain that.
Andrew
Well, it’s the wavelength, not amplitude. This seems to be a common point of confusion, probably because an EM wave is often diagrammed as traveling straight on one axis, with the E-field shown on the Y-axis and the H-field on the X-axis (like in the second link below). Those are simply the field strength and have nothing to do with the way it ‘looks’ in space.
Here’s a simple statement of the same thing -“How does a Faraday Cage work?” (Note that as AndrewT points out, a Faraday Cage is actually different; that article uses the term somewhat loosely for shielding).
As to how EMI shielding in general works, I can’t come up with a good explanation. You can kind of say the wave doesn’t ‘notice’ the shield if the hole is too big, but it’s more complicated than that. One thing, though - the holes ought to be a fraction of a wavelength for it work well. Here’s a decent article on shielding, though it is highly technical.
Some wireless keyboards, and mice, use RF rather than infra-red light.
I wouldn’t be surprised if they’re going to pretty much all RF. I know I’ve got an RF Logitech mouse and keyboard, because I have the reciever unit practically on the ceiling and it works just fine.
All the one’s I seen in the last few years have been using RF. But I’ve seen infra-red models too. The IBM PC-jr. came with a infra-red linked keyboard. Where I work, a major aerospace manufactorer, RF keyboards are prohibited due to the possability of interception.
But infra-red light doesn’t have to be as carefully monitored by the FCC (I think) because it doesn’t go all over the place, the way “radio waves” do. Try pointing the remote away from the TV, or blocking the IR beam with a solid object. Your “channel up” signal isn’t going to get outside the walls of your house.
IR reflects off of most walls. I can control my TV by pointing my remote in pretty much any direction in my basement. Just because you can make it work without pointing directly at the receiver doesn’t necessarily mean that it’s RF.
However, I just checked a couple of web sites and it does seem that the majority of wireless keyboards and mice are using RF these days.
thanks e_c_geek, that analogy helps a lot
and the chart, ccwaterback, makes it easy to see how they are divided up. thanks.
from ccwaterback’s link to the chart (at bottom of linked page), it appears that all the frequencies are taken up.
can there be no more used?
and can a certain frequency used (like cell phones) have a limit to the number of devices using it? (for example, can 10 billion cell-phone signals saturate the frequency to the point where no more can be added?)